In a conventional drilling operation, once the well has been drilled, it is completed by placing a liner or casing to support the well and provide zonal isolation. The casing is typically a steel pipe that is run into the well and located by placement of cement around the outside of the pipe to provide a seal between the pipe and the underground formation. Communication between the formation and the inside of the casing is achieved using explosive charges to perforate the casing at the desired locations. This technique is relatively straightforward where the well is vertical or close to vertical and has a relatively wide diameter. In this case, the casing can be run into the well under its own weight and there is usually sufficient clearance in the annulus for cement to be pumped down the casing and back up to the surface.
Where the well is highly deviated from vertical, especially when it is close to horizontal, placement of completion equipment can be more complex. In the deviated section, the effect of gravity in moving the equipment towards the bottom of the hole is greatly reduced (and is completely absent in a horizontal hole). Furthermore, drag due to contact between the casing and the borehole wall is greatly increased. If the deviated section is relatively short and/or it is preceded by a relatively long vertical section, the weight of casing in the vertical section is usually enough to force the part in the deviated section into position. However, there are times when placement in such a manner is not possible. In these cases, the end of the borehole is left open (sometimes called “barefoot completion”). In cases in which the formation is unstable or weak, this may cause problems due to hole collapse, wash out or sanding.
Recently, no-rig drilling techniques, such as coiled tubing drilling or wireline drilling have been proposed (as are discussed, for example WO 2004072437 A). Such techniques are often proposed for drilling highly deviated wells or sidetracks from existing wells. As well as the problems identified above for the placement of completion equipment in deviated wells, such techniques also suffer from the problem that there is no casing in the vertical section to force the completion equipment into the deviated well or sidetrack. This, combined with the high degree of deviation and small diameter common to such techniques mean that the drag is often too great to allow proper placement of completion equipment.
In conventional, rig-based drilling, one extreme form of drag is encountered when the drill pipe becomes stuck. Drillers often try to overcome this problem by the use of drilling jars placed in the drill string and operated to apply axial shocks in upwards and/or downwards directions to unstick the drill pipe. Drilling jars typically comprise a sliding mandrel in a sleeve. In use, the mandrel is driven up or down by some form of stored energy, a hammer on the mandrel striking an anvil on the sleeve so as to impart a shock and (it is hoped) free the stuck pipe. The use of drilling jars is discussed in more detail in ASKEW. Jars, Jarring and Jar Placement. Oilfield Review. Oct. 1991, p. 52-61. One common form of drilling jar is a hydraulic jar. A hydraulic jar consists of two reservoirs of hydraulic fluid separated by a valve. When tension or compression is applied to the tool in a cocked position, fluid from one chamber is compressed and passes through the valve at high flow resistance into the second chamber. This allows the tool to extend or contract. When the stroke reaches a certain point, the compressed fluid is allowed to suddenly bypass the valve. The jar trips as the fluid rushes into the second chamber, instantly equalising pressure between the two chambers and allowing the hammer to strike the anvil. The greater the force on the jar, the sooner and more forceful the release. Examples of commonly-used drilling jars are the PowerTrac jar and the Earthquaker and Hydraquaker jars.
Such techniques are not typically applicable do to the absence of the drill string to apply a force to cock and trip the jar, such as, for example, in the context of drilling with a wireline machine.